Traditional filter presses consist of a series of filter plates, each covered by filter cloth, supported in substantially parallel relationship on bearers and pressed together by mechanical means, such as a movable head and a screw spindle. Spaces are formed between adjacent plates due to raised rims around their edges, or by insertion of spacers in the form of hollow frames. These spacers are sealed tight by mechanical pressure during operation and then filled with suspension to be filtered, the filtrate being forced through the cloth, and via grooves in the plate to an outlet. Generally, there is a tendency to produce thick filter cake with such apparatus, e.g. 12 mm on each plate face, in order to increase the yield per cycle. However, this reduces efficiency as resistance to draining increases exponentially with cake thickness, and high pressures are therefore required. Additionally, cake discharge necessitates manual intervention, such as the pushing apart of plates followed by hosing down or scraping, and it is often problematic.
A development from the traditional filter press has been use of a closed pressure vessel, as outlined in the first paragraph above. One form of such apparatus is known as a Kelly filter and it has the unit of filter plates mounted on a carriage for movement in and out of the outer shell or vessel. Again, care discharge is problematic. Dry filter cakes cannot practically be produced since they would compact the entire filter plate unit and caused it to be damaged upon removal from the shell as well as being very difficult to remove.